Matched LED

Matched LED

Hello,

I'd like to implement this envelope filter:

* w w w .generalguitargadgets . com /pdf/ggg nurse quacky.pdf
In the part list "2 matched leds" are needed.

1) which are the requirements for matching 2 leds?
2) does a way exist to determine a couple from a bunch of - say - 30
of them ?
3) is the color (yellow, red, green) important ?

Thanks in advance for the answers

Bye

Re: Matched LED

"grunged" <groundgrunge@hotmail . it > schrieb:

>Hello,
>
>I'd like to implement this envelope filter:
>
> * w w w .generalguitargadgets . com /pdf/ggg_nurse_quacky.pdf
>In the part list "2 matched leds" are needed.
>
>1) which are the requirements for matching 2 leds?

same forward voltage

>2) does a way exist to determine a couple from a bunch of - say - 30
>of them ?

multimeter with diode-tester. if you buy good led's (no low-cost), the
diference will be less. you may only need 15.

>3) is the color (yellow, red, green) important ?

yes, different collor = different forward voltage.

>
>Thanks in advance for the answers
>
>Bye
--
Mailadress: klein AT technik-klein DOT de
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Re: Matched LED


"Juergen Klein" <see_text@technik-klein.de> wrote in message
news:fcvf03petpfkcn56hbp7918usb44cm0c5e@4ax . com ...
> "grunged" <groundgrunge@hotmail . it > schrieb:
>
>>Hello,
>>
>>I'd like to implement this envelope filter:
>>
>> * w w w .generalguitargadgets . com /pdf/ggg_nurse_quacky.pdf
>>In the part list "2 matched leds" are needed.
>>
>>1) which are the requirements for matching 2 leds?
>
> same forward voltage
>
>>2) does a way exist to determine a couple from a bunch of - say - 30
>>of them ?
>
> multimeter with diode-tester. if you buy good led's (no low-cost), the
> diference will be less. you may only need 15.
>
>>3) is the color (yellow, red, green) important ?
>
> yes, different collor = different forward voltage.
>
>>
>>Thanks in advance for the answers
>>
>>Bye
> --
> Mailadress: klein AT technik-klein DOT de
> Advertisement to this mail address is prohibited!
> Home: * w w w .underwood.de.vu
> Ebay: * w w w .underwood.de.vu/ebay.php

The best way to match the LEDs is to set them up in a test circuit. Look at
the LED used to bias the opamp. It runs 9V through a 10K resistor and to
ground. Set up a test circuit exactly the same, and measure the voltage at
the LED. It will be about 2 volts. Find two that are closest together. It
really should have a small filter cap across it because the diode junction
tends to be a bit noisy. It will set the positive input at about 2V above
ground. This will cause the outpout voltage to be at the same level so
theoritically, there is no current out of the output of the opamp without an
input signal. If the LEDs don't match, then there might be some current
flowing at all times, or it might take more of an input signal to get
current to flow. I don't think it really matters, because until the voltage
out of the opamp exceeds the forward foltage of the LED plus the
base-emmitter forward voltage of the transistor (about .7V), the transistor
won't turn on anyhow. Is this by design?